Small Matters

Millions of years ago, leafcutter ants learned to grow fungi. But how? And why? And what do they have to teach us?

A few decades later, the great English naturalist Henry Walter Bates, noting the leafcutters’ industry and grace in his 1863 masterwork, The Naturalist on the River Amazons, also recorded a more baleful view—that of local farmers who considered the ants a “terrible pest.” But Bates, like other observers, thought the leafcutters used their cuttings as protection from rain. (They are also called parasol ants.) It took an engineer and self-taught ecologist, Thomas Belt, to finally figure out how the leaves were actually used. On a mining operation in Nicaragua, Belt excavated two leafcutter nests. To his surprise, he could find few signs of cut leaves. Upon close examination of the spongy brown material filling the chambers, however, Belt noticed “minutely subdivided pieces of leaves, withered to a brown color, and overgrown and lightly connected together by a minute white fungus. . . .” The leafcutters, Belt wrote in 1874, “are, in reality, mushroom growers and eaters.”

Not long after, William Morton Wheeler, the dean of ant research at Harvard, wrote an entire book on the fungus growers. And Edward O. Wilson, who would later succeed Wheeler as the preeminent ant scholar at Harvard, dubbed leafcutters “among the most advanced of all the social insects.”

But researchers trying to better understand Belt’s breakthrough observations faced major obstacles, particularly when it came to identifying the kind of fungi the ants were growing. Scientists typically identify a fungus through its sporophore, the part of the plant that produces spores. In ant gardens, however, the sporophores are rarely in evidence for reasons that remain unclear. “It’s as if the ants have castrated the fungus,” Schultz explains. (In essence, the ants propagate the fungi by taking cuttings.) Lacking a method for identifying fungus types, scientists were missing half the story.

This is where things stood when mueller and Schultz first crossed paths at CornellUniversity in the late 1980s. There, they teamed up with fungus specialists Ignacio Chapela, now at the University of California at Berkeley, and Stephen Rehner of the U.S. Department of Agriculture in Beltsville, Maryland. Chapela pulled individual strains of fungi from the ant gardens and kept them alive. Using molecular genetics techniques, Rehner then described the differences between the various strains. Schultz matched those results with his DNA analysis of the associated ants. In 1994, the foursome published a study in Science magazine documenting the interaction between fungi and the ants. “It is now clear,” they wrote, “that the origin of the fungus-growing behavior was an extremely rare event, having occurred only once in the evolutionary history of the ants.” The most sophisticated attines, the researchers surmised, had propagated one fungus lineage for at least 23 million years.

In a follow-up report four years later, Mueller, Rehner and Schultz modified the accepted wisdom, arguing that attine fungi often represented a variety of species—not just one passed along by founding queens from nest to nest. More “primitive” attines, the scientists wrote, sometimes share their fungus with one another, even with distantly related ant species—a version, the biologists suggested, of crop rotation. “We can show that crop failure is a major issue in their lives,” Mueller explains. “They do the same thing that humans have done, going to the neighbors to find a replacement, sometimes stealing it, sometimes overrunning and killing the neighbors, too. We’ve shown this in the lab. The next logical thing is to look for it in the wild.”

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